Conventional over current circuit breakers are normally so set to cut off the power supply when the current through an electric system exceeds a predetermined level which is normally set at 1.5 to 2 times the rated capacity of the electric system. However, if the electric system involves induction motors, the system often draws a relatively large amount of electric current, which is normally regarded as a starting current, which acts as a surge, at the starting period, usually within 15-20 seconds after the system is turned on. The starting current is often as large as 5-9 times the rated capacity of the system and would cause the over current circuit breaker to "trip", if the over current circuit breaker were set at 1.5 to 2 times the rated capacity of the system, and so the system will never operate normally. If the over current circuit breaker is set at a level to allow the system to draw an electric current as large as the starting current, the over current circuit breaker will not "trip" even when the system is overloaded or a short-circuit takes place, and will thus fail to protect the system from damage.
In view of the aforesaid problems with conventional over current circuit breakers, the present invention offers a highly efficient over current circuit breaker which will not "trip" when the electric system is turned on but will "trip" when the system is overloaded or a short-circuit takes place.